1. Field of the Invention
This invention relates to diaphragm-type vacuum pump devices for generating a required vacuum by a displacement of a diaphram operated by a motor and, more particularly, to diaphragm-type vacuum pump devices used in, for example, a vacuum-actuated speed control system or a vacuum-actuated brake booster in a motor vehicle, as a vacuum source to supply vacuum under a condition where insufficient engine vacuum is generated at an engine intake manifold.
2. Discussion of the Background
In a conventional diaphragm-type vacuum pump device such as shown in Japanese Utility Model Laid-open Application No. 50 (1975)-155610 or Japanese Utility model Publication No. 58(1983)-36867, the diaphragm-type vacuum pump device generates a required vacuum by reciprocating motion of a diaphragm when the vacuum at the engine intake manifold decreases and is less than a predetermined value.
Such diaphragm-type vacuum pump device uses, for example, a vacuum-type speed control system as shown in FIG. 3 in which the actual speed of the motor vehicle is controlled automatically and is automatically maintained at a set speed without depressing an accelerator pedal.
In the vacuum-type speed control system, an engine intake manifold 1 is connected to a chamber 10a of a vacuum pump via a pipe 2. As a result, a valve 11 is changed to an open condition by the vacuum existing in the engine intake manifold 1. Then the vacuum at the engine manifold 1 is communicated with an actuator 5 via the valve 11, a chamber 10b (communicable with the chamber 10a through the valve 11) and a pipe 4. The actuator 5 pulls an accelerator link by the force of the atmospheric pressure caused by the vacuum at the engine intake manifold 1. In order to pull an accelerator link by the actuator 5, the vacuum level at the engine intake manifold 1 is higher than a predetermined required level, (i.e., a mm Hg) as represented in FIG. 4. The actuator 5 is operated by a controller 6 and controls the degree of opening of a throttle valve 7.
When the vacuum at the engine intake manifold decreases and reaches the predetermined level, a vacuum responsive switch 8 operates and supplies a detecting signal to the controller 6. Then the controller 6 supplies a current to a motor 12 which drives the vacuum pump 10. The motor 12 operates to rotate a crank shaft of vacuum pump 10. A diaphragm 13 reciprocates vertically in response to the movement of the crank shaft. Therefore valves 14 and 15 are respectively alternately changed to opposite conditions. Thus the vacuum at the chamber 10b is increased.
The vacuum pump 10 is required to generate a vacuum higher than the determined pressure (a mm Hg) even when flow consumption is at a maximum value. Therefore, the vacuum pump having pumping characteristics as shown in FIG. 4 is used.
However, in the conventional diaphragm-type vacuum pump 10, when flow consumption in the actuator 5 is less, the generating vacuum in the vacuum pump 10 is increased as shown in FIG. 4. Furthermore, in starting the vacuum pump 10, a heavy load is applied to the motor 12. Therefore, a large starting torque in the motor 12 as a driving power source of the vacuum pump 10 is required. Consequently, in the conventional vacuum pump, the motor for driving the vacuum pump is required to be more powerful and must be more durable.